The present invention relates generally to efficient use of light output in a backlight for a liquid crystal display device, and particularly to minimization of light lost to internal reflectance.
Obtaining the maximum light energy output for a given power input to a fluorescent lamp used a backlight in an active matrix liquid crystal display (AMLCD) is an important operational feature. In particular, AMLCD devices transmit very little of the backlight provided. For a color AMLCD, only 2.5% to 4% of the backlight passes through the AMLCD. For monochrome applications, up to 12% of the backlight passes through the liquid crystal display (LCD). In either case, the most efficient extraction of light from the backlight must be achieved to maximize the light output from the display device for a given power input. The lumens (light out) per watt (power in) conversion in a LCD backlight system can be taken as a measure of efficiency for a fluorescent lamp backlight system. Minimizing light loss improves this measure of efficiency.
As a result of inherent limitations in the AMLCD, the viewing angles are generally restricted in both vertical and horizontal directions. Consequently, it is desirable to restrict, as much as possible, the visible light produced within given horizontal and vertical view angles such that a user of the LCD device receives the maximum available light when observing the display within the view angles. The result is improved contrast in images presented on the LCD device. It is desirable, therefore, to redirect light which would otherwise exit beyond the view angles to minimize losses resulting from absorption inside the housing. Prior engineering efforts have attempted to develop diffuse, uniform illumination backlighting for AMLCDs. In conventional backlight schemes, a diffused light from the backlight is generally emitted into a very wide cone, much larger than the viewing cone typically defined by the horizontal and vertical viewing angles of the AMLCD. Light emitted from the backlight at angles between the defined viewing angles and 90 degrees to the display normal is not used efficiently to produce viewable luminance on the face of the flat panel display. Accordingly, a larger portion of the light emitted in these regions is unavailable to the viewer.
Prior methods of optically redirecting the light output of the backlight include Fresnel lenses and non-imaging optical reflectors. Fresnel lenses offer good diffusion, but light is lost due to spacing between the lenses and the directional capabilities are not readily controlled. Non-imaging optical reflector arrays can offer good direction and efficient performance for a single fluorescent lamp tube. However, "dead bands" occur at the reflector junctions when a larger area is to be illuminated with multiple lamp legs. This is highly undesirable for flat panel display applications which require uniform illumination over a large surface.
Directional gain via prismatic refraction may be provided by use of Scotch.TM. optical lighting film (SOLF) which operates on the principal of total internal reflectance. The SOLF requires the use of a supplementary filter or reflector to diffuse light before redirecting it over the target area. SOLF is normally manufactured with 45 degrees V-grooves running in one direction.
It is desirable, therefore, that an LCD display device make more effective use of the light produced by a light source used as a backlight by directing more of the available light within given viewing angles of the display such that the light energy otherwise lost by emission outside of the AMLCD viewing angle is directed within the field of view of the display.